1. Field Of The Invention
The present invention relates to thermal flow sensors which employ thermosensitive resistors to detect the flow rate of fluids such as gases.
2. Description Of The Related Art
A flow sensor is known which detects the flow rate of a fluid from a balanced state of a bridge circuit including a thermosensitive resistor disposed in the fluid.
FIG. 1A is a front sectional view of a conventional thermal air-flow sensor, and FIG. 1B is a side sectional view of the sensor shown in FIG. 1A. Referring to these figures, the sensor has a housing 1 defining the main passage for the fluid, and a supporting pipe passage 2 provided substantially in the center of the housing 1. A thermosensitive resistor R.sub.H which comprises, for instance, a small platinum thin film resistance is provided within the supporting pipe passage 2 for converting a change in its temperature caused by the fluid into a corresponding change in its resistance, and an air temperature sensor R.sub.C is also provided within the supporting pipe passage 2 for detecting the air temperature and thus enabling the correction of any variation in the change in temperature of the air. Leads 3 and 4 are provided for the thermosensitive resistor R.sub.H and the air temperature sensor R.sub.C, respectively. The leads 3 and 4 are passed through holes (not shown) formed in the housing 1 and the supporting pipe passage 2, and are connected to a control circuit 5 provided on the outer periphery of the housing 1.
FIG. 2 is a circuit diagram showing a bridge circuit including the thermosensitive resistor R.sub.H and the air temperature sensor R.sub.C, and also showing a control circuit for effecting control in such a manner that the bridge circuit is maintained in a balanced state. The bridge circuit comprises resistors R.sub.1 and R.sub.2, in addition to the thermosensitive resistor R.sub.H and the air temperature sensor R.sub.C. The bridge circuit has junctions b and f connected to the input terminals of a differential amplifier 6 the output terminal of which is connected to the base of a transistor 7. The emitter of the transistor 7 is connected to one terminal a of the bridge circuit, and its collector is connected to the anode of a DC power source 8. The fluid flows in the direction indicated by an arrow 9.
With the above-described conventional air-flow sensor, fluid rate is detected in the following manner. Before a fluid is allowed to flow, a certain voltage is applied to the bridge circuit to cause the thermosensitive resistor R.sub.H to generate heat. When the fluid i.e. air, comes into contact with the resistor R.sub.H, the temperature of the resistor R.sub.H drops, thereby causing a change in the resistance of the resistor R.sub.H. A current I.sub.H increased by an amount corresponding to the change in the resistance of the resistor R.sub.H, i.e. to the flow rate of the fluid, is caused to flow through the resistor R.sub.2, thereby bringing the bridge circuit into a balanced state. In this balanced state, the voltages at the junctions b and f are equal. On the basis of the current I.sub.H, the voltage V.sub.O =I.sub.H .times.R.sub.2 at the junction b is extracted as the flow rate of the fluid from a signal outputting portion S. A signal from the signal outputting portion S is fed to a signal processing section, not shown, then processed therein, so as to determine the flow rate of the fluid. Because changes in air temperature cause variations in the change in resistance of the resistor R.sub.H, the variations are corrected by the use of the air temperature sensor R.sub.C.
The above-described conventional thermal flow sensor, however encounters the following problem. Since the flow of fluid within the housing 1 is not uniform, and it may be dense or coarse within the same plane, depending on such factors as the configuration of the upstream piping, there is a risk that the flow rate may not be detected correctly if the flow rate is only detected at one position by the use of a single thermosensitive resistor R.sub.H, as in the conventional thermal flow sensor.